This invention relates to transimpedance amplifiers and other circuits which incorporate current-to-voltage conversion. More particularly, the present invention relates to circuits and methods for rejecting low frequency signals in current-to-voltage transimpedance amplifiers to ensure that signals from ambient sources, for example, sunlight interference with the operation of a photodiode, can be fully attenuated.
The use of photodiodes as receivers for data transmission and as sensors, is well known. In typical photodiode applications, incident light received by the photodiode is transformed into a current. The amount of current produced by the photodiode indicates the intensity of light received by the photodiode.
The output current of the photodiode is often converted into a voltage, which is amplified and compared to predetermined threshold values. The comparison may result in a decoding process which converts the varying light into a stream of data bits, such as an infrared data transmission system in an office environment. In the case where the photodiode is merely being used as a sensor, the comparison may simply indicate that a specific event has occurred (for example, that a manufactured part has passed a certain point along an assembly line).
Various difficulties may be encountered when attempting to utilize photodiodes. In particular, ambient light may interfere with the reception of light from the signal source. For example, in the office environment discussed above, several other light sources in the office may provide interfering light to the infrared system, including sunlight, incandescent lamps, and fluorescent lights. In most cases, it is desireable to filter out the ambient light from the light signal. The filtering is typically accomplished through the use of low-pass and high-pass filters. While the use of low-pass filters (to eliminate light having frequencies higher than the light signal) is relatively straightforward to implement, various difficulties occur in attempting to filter out signals at frequencies lower than the light signal.
One of the most basic solutions for a current-to-voltage converter including a low-pass filter is shown in FIG. 1. As described above for multiple photodiode configurations, it is preferable to have the current-to-voltage circuit on the same silicon chip as the photodiode to eliminate problems commonly experienced through the use of discrete components (e.g., leakage current errors and noise pickup).
The circuit of FIG. 1 includes photodiode 102 (shown schematically as current source 104 and junction capacitance 106), which receive light from at least light source 108, resistors 110 and 112, capacitor 114 and amplifier 116. Resistor 110 converts the current passing through photodiode 102 (I.sub.PD) into a voltage which is amplified by amplifier 116 into output voltage V.sub.OUT. The low-pass filter is the RC network formed by resistor 112 and capacitor 114, which has a cutoff frequency that is set by the values of resistor 112 and capacitor 114.
One inherent deficiency of the circuit of FIG. 1 is the low sensitivity of the circuit due to the fact that resistor 110 must have a low resistance value. The requirement for low resistance in resistor 110 occurs because resistor 110 and capacitor 106 also form a low-pass filter which, as resistor 110 is increased to improve sensitivity, may act to block out the desired signals.
One attempt at resolving these deficiencies is the replacement of amplifier 116 and resistor 112 with an operational amplifier (opamp) which performs the transimpedance function. In such a circuit a resistor and capacitor, which may be configured in a similar manner as shown by resistor 110 and capacitor 114 of FIG. 1, are used to set the high pass frequency. However, the circuit may be particularly susceptible to noise because of the conflicting requirements between the resistor and the capacitor.
For proper performance, the capacitor must be large as compared to the photodiode's capacitance, and the resistor must be large to minimize low frequency noise. However, if both elements are large, a high break frequency is simply not possible. Further, if the resistor is large and the capacitor is small, the gain of the circuit is reduced and the input referred noise is large, resulting in additional degradation of sensitivity.
Another attempt at resolving the above described deficiencies is shown in FIG. 2. In FIG. 2, the output current from the photodiode 102 is input to the inverting terminal of opamp 202, which provides the transimpedance function. The break frequency of the circuit is set by resistors 204 and 214, capacitors 206 and 216, and the ratio of resistor 208 to resistor 210. A non-inverting integrator 212 is coupled to the output of opamp 202 (through resistor 204) to force the output of opamp 202 (V.sub.OUT) to be zero at frequencies below the break frequency. For proper operation, the product of the values of resistor 204 times capacitor 206 should substantially equal the product of the values of resistor 214 and capacitor 216.
The loop from opamp 202 to integrator 212 and back forces V.sub.OUT to be zero at sub-break frequencies by the current which flows through resistor 208 based on the output voltage of integrator 212. At high frequencies (i.e., above the break frequency), capacitors 206 and 216 are short-circuits which essentially turns off integrator 212. While this circuit provides output signals with relatively high sensitivity, changes to the break frequency may be difficult to accomplish because at least two elements must be changed. For example, if capacitors 206 and 216 have fixed values, resistors 204 and 214 must be adjusted to change the break frequency. Further, changing the two resistors will require access to at least three pins on the integrated circuit containing opamp 202.
In view of the foregoing, it would be desirable to provide transimpedance amplifier circuits in which the break frequency may be adjusted through the use of a single circuit element.
It would also be desirable to provide transimpedance amplifier circuits in which the break frequency may be adjusted via a single interface to the amplifier circuit.
It would be further desirable to provide transimpedance amplifier circuits which reject low frequency signals and allow high frequency signals to pass unaffected thereby providing high sensitivity.
It would be still further desirable to provide methods for adjusting the break frequency of a transimpedance amplifier circuit with minimal effects to the interface between the amplifier circuit and external circuitry.